1. Field of the Invention
The present invention relates to a method of producing a semiconductor device, and more particularly, to an ion implantation method of writing information in a Read Only Memory (ROM), etc.
2. Description of the Relate Art
As a method of writing information in a ROM, a that of controlling the threshold voltage (Vth) of a MOS transistor is well known. In this ROM, each cell is composed of the MOS transistor or a resistance.
Conventionally, the control of the threshold voltage (Vth) is usually carried out before the gate electrode formation during the MOS transistor production process, due to the limitations (usually 200 KeV or less) of the accelerating energy of the ion implanting apparatus used to introduce impurities into a semiconductor.
Recently, however, an ion implanting apparatus having a high accelerating energy of, e.g., 0.4 to 3 MeV, has been actually used and control of the Vth realized thereby, through a gate electrode and/or an insulating film positioned thereon.
This manner of writing information in a ROM shortens the period (turnaround time) from receipt of an order to shipment of the device. Conventional examples thereof are now explained.
FIGS. 1A to 1C are cross-sectional process views of a conventional method of implanting ions in a ROM.
As shown in FIG. 1A, a p type silicon substrate 1 is provided with n.sup.+ source/drain regions 2 and 3 and a gate electrode 4 having a 200 .ANG. thick gate silicon oxide film 5 formed thereon.
Then, as shown in FIG. 1B, after forming a patterning mask 7 by which the gate electrode 4 is exposed, ROM information is written by implanting n type impurity ions, for example, P.sup.+, in the silicon substrate 1 through the gate electrode 4 and the gate silicon oxide film 5 with an energy of about 100 to 200 KeV, so that an n-type region having a depth of about 0.2 .mu.m or less is formed.
Thereafter, the usual processes, for example, a wiring or interconnection forming process and a passivation process, are carried out as shown in FIG. 1C, and a final ROM product, is produced. In FIG. 1C, 8 denotes a passivation film, etc. In this process, however, the turnaround time from information writing to final product is a lengthy period of one week or more.
FIG. 2 shows a cross-sectional view of another conventional example, wherein impurity ions having a high energy are implanted, from above insulating films (passivation film, etc.) 8 formed on a gate electrode 4, in a p-type silicon substrate 1.
Namely, as shown in FIG. 2, n-type impurities, e.g., P.sup.+, having a high energy of 0.4 to 3 MeV are implanted from above the passivation films etc. 8 formed on a gate electrode 4, in a p type silicon substrate 1. This high energy implanting method shortens the turnaround time from the ion implanting process to the final product to, for example, two or three days. However, since the implanting distance of ions is lengthened by using the high energy implanting apparatus the ion spread in the lateral direction is made wider (0.7 .mu.m in FIG. 2) and the widened region has an adverse influence on an adjacent cell when information writing in a cell. This problem has become more serious with the need for a shrinkage of cell in a ROM. Note, when the ion implanting energy is increased, the ion-spread in the lateral direction is greater, and thus, it is difficult to shrink ROM cells beyond a certain limit.
For example, when implanting ions through a SiO.sub.2 film or a Si film having a thickness of 1.5 .mu.m, a width of about 0.2-0.4 .mu.m occurs to a region having a concentration one-tenth that of the implanting region.
To solve the above-mentioned problem of the widening of ion implanted region in the lateral direction, the present inventor proposed an ion implanting method disclosed in a Japanese Unexamined Patent Publication (Kokai) No. 63-299119 wherein, during a high energy ion implantation, opposite conductive type impurity ions (compensating ions) having a wider region in the lateral direction than that of the writing ions are implanted at a low dose from the opening of the same mask as used for the writing, so that the conductive type is compensated at a tail portion.
Although, this ion implanting process allows a greater shrinkage of cells, the opposite conductive type ions were restricted such that they satisfy only the above-mentioned conditions with respect to the writing ions.